On April 8,  U.S. Environmental Protection Agency (EPA) Administrator Michael S. Regan signed a final rule regulating six per- and polyfluoroalkyl substances (PFAS) under the Safe Drinking Water Act (SDWA). The final rule, which will become effective 60 days after publication in the Federal Register, sets individual maximum contaminant levels (MCLs) for PFOA and PFOS at 4.0 nanograms per liter (ng/L), and for PFHxS, PFNA, and HFPO-DA at 10 ng/L. In addition to these individual MCLs, the rule establishes a hazard index (HI) of one (unitless) as the MCL for any mixture containing two or more of PFHxS, PFNA, HFPO-DA, and PFBS. Under the rule, regulated public water systems (PWS) must conduct and report their initial monitoring within three years after publication, and make any necessary capital improvements to comply with the MCLs within five years after publication.